This invention relates to a supply circuit for supplying a smoothed direct current to a load impedance, comprising:
input terminals for receiving a voltage consisting of a direct voltage component and a periodic alternating voltage component superposed thereon having a specified ripple period,
output terminals for connecting the load impedance,
a series element controllable via a control input and having a main current path connected in series with the load impedance,
a low pass filter having an output which provides a voltage with a direct voltage component determined by at least one filter capacitor,
and a differential amplifier having an output connected to the control input of the series element, having a first input to which is supplied a voltage proportional to the voltage across the load impedance, and having a second input connected to the output of the low pass filter.
Such a supply circuit is known from "Patent Abstracts of Japan", publication No. 59-170915, publication date 27-9-1984, application No. 58-44442, date of application 18-3-1983.
The supply circuit described in the aforesaid publication comprises a differential amplifier whose output is connected to the base of a bipolar transistor which forms the series element. A first input of this differential amplifier is connected to the tapping point of a voltage divider connected between the output terminals of the supply circuit, and the second input of this differential amplifier is connected to the tapping point of a further voltage divider connected between the input terminals of the supply circuit. Arranged in parallel with a resistance of this further voltage divider is a capacitance, through which an alternating voltage component of the input voltage is short circuited. Thus the input voltage divider together with this capacitance forms a low pass filter. Because of this low pass filter the alternating voltage component has no effect on the output voltage of the differential amplifier, thus on the base voltage of the bipolar transistor. In order that there be no alternating voltage component across the load impedance, the entire alternating voltage component has to appear across the series transistor. For this to be achieved the direct voltage between the collector and the emitter of this series transistor should necessarily be selected equal to at least the peak-to-peak value of the alternating voltage component. This can be achieved by selecting the right resistance values of the two voltage dividers.
This arrangement results in a considerable power dissipation occurring in the series transistor, irrespective of the real amplitude of the alternating voltage component. Consequently, this series transistor continuously operates at a high working temperature leading to fast ageing and a short life of the transistor. This is especially a drawback for systems where high demands are made on the operational reliability, such as for example telecommunication exchanges.